Chi published the photos of the 31-year-old Middleton wearing a blue bikini that showed a barely rounded lower belly. Middleton and Prince William were on vacation on the exclusive Caribbean island of Mustique in St. Vincent and the Grenadines when the photos were taken.

"We are disappointed that photographs of the duke and duchess on a private holiday look likely to be published overseas," a spokesman for the duke and duchess told ABC News. "This is a clear breach of the couple's right to privacy."

Chi is published under the Mondadori publishing house owned by former Italian Prime Minister Silvio Berlusconi. Chi is also the same magazine that, in 1997, published photos of Princess Diana's dying in a tunnel in Paris after a high-speed car chase with paparazzi that ended in her death at age 36.

"Everybody is entitled to privacy, but it's just different when you're a royal," said Victoria Arbiter, royal contributor for ABC News. "They all crave normality, but they're not just like everybody else."

The British media, as well as ABC News, have declined to publish the new photos of a pregnant Middleton but today the Australian magazine Woman's Day says it will follow Chi's lead and publish the pictures.

"It wasn't a hard decision to run these photos," said the magazine's editor, Fiona Connolly. "Kate is on a public beach. There are other holiday-makers there and she looks fabulous."

The island where the royal couple was vacationing, Mustique, prohibits paparazzi. Middleton and Prince William were also staying in a private villa on private property when the photos were taken.

Middleton, pregnant with the couple's first child and expected to give birth in July, has only recently begun to show her baby bump after a difficult start to her pregnancy.

Buckingham Palace was forced to announce the pregnancy in December after Middleton was admitted to the hospital, where she spent four nights, for hyperemesis gravidarum, an acute morning sickness that requires supplementary hydration and nutrients, the palace said.